In modern digital mobile systems, traffic channels are capable of setting up a circuit switched data link by means of which a data terminal equipment that is connected to a data interface of a mobile station can transmit data to and from an interworking function unit (such as a data modem) placed on the mobile network side (e.g. at a mobile services switching centre) and further, by means of a fixed network data link, to and from another data terminal equipment. The maximum transmission rate for such data transmission is usually determined by the maximum capacity of the data channel set up over the radio path.
Data compression is widely used to increase the efficiency of data transmission. Possible redundancy is removed from a user data stream with a data compression algorithm at the transmitting end, which results in a reduced amount of data to be transmitted. At the receiving end, a decompression algorithm expands the user data back into its original form. Typical maximum compression ratios provided by the compression algorithms are in the range from 2:1 to 4:1, which enables the adaptation of a user data stream of up to 19200/38400 bps into a data channel of 9600 bps in a cellular radio system. However, the actual compression ratios provided by the compression algorithms are highly dependent on the type of the user data. There are several standard or de facto standard data compression methods. For example, data modems usually support ITU-T V.42 bis and MNP5 methods. In the digital GSM mobile system, for example, the recommendations determine that the ITU-T V.42 bis compression method is used between a mobile station MS and an interworking function IWF.
Data compression methods usually require completely error-free data transmission since even the smallest transmission error confuses the decompression algorithm at the receiving end. The ITU-T V.42 bis, for example, is based on dynamic build-up of compression trees according to specific rules at both the transmitting and the receiving end. If uncorrected transmission errors occur, the trees will develop differently and the data will be corrupted. Therefore, error correction that is as efficient as possible must be used over the entire data link in order to prevent the occurrence of transmission errors.
In modems, the error-correcting layer below the V.42 bis data compression is the ITU-T V.42 (LAPM, link access protocol for modems). In digital transmission through a PSTN (public switched telephone network) or an ISDN (integrated services digital network), the error-correcting layer below the V.42 bis data compression may be for example a V.120 protocol that operates in a multiframe mode (in other words the error correction is based on the retransmission of corrupted frames). The aforementioned error correction protocols are designed for error conditions that are typical of fixed lines, but they are insufficient or inappropriate for special conditions, such as a radio link. Therefore, it has been necessary to implement special error correction arrangements within a mobile system. For example in the GSM system, the error-correcting layer below the V.42 bis data compression is the radio link protocol RLP, which is also based on the retransmission of frames.
The use of data compression must be agreed on somehow and possible compression parameters must be negotiated before data transmission may begin. In the GSM system, both parties, i.e. a mobile station MS and a mobile services switching centre MSC, indicate their data compression support in call set-up signalling. Further, after the RLP has been set up, an inband negotiation takes place between the MS and the IWF by means of XID frames of the RLP. An inband negotiation takes place over a modem connection by means of XID frames of a V.42 protocol after the V.42 error correction protocol has been set up between the modems. An inband negotiation takes place over a V.120 connection by means of XID frames of a V.120 protocol after the V.120 error correction protocol has been set up.
In practice, there are data links that consist of physically and protocolwise separate legs, for example a GSM data call through a PSTN (the connection consists of two legs: 1) a GSM traffic channel with its protocols and 2) a modem connection through the PSTN) or a GSM data call through an ISDN (two legs: 1) a GSM traffic channel with its protocols and 2) an ISDN protocol, e.g. V.120). In such a case, there are two possibilities of providing data compression over the entire connection: 1) Separate compression on each leg (e.g. as defined currently in the GSM recommendations) and 2) end-to-end compression (as implemented by some mobile phone manufacturers in the GSM system).
Each embodiment has problems. In case 1), a high processing power is required in the IWF situated between the different legs for performing data compression in both directions on two different legs. Further, the IWF requires a large memory to support the compression trees of two different compression units. Between the compression units there must be a high speed interface so that uncompressed data can be transmitted from one compression unit to another.
End-to-end compression (case 2) can be implemented in such a way that the calling party requests for a transparent synchronous connection in order to handshake the error correction and data compression protocol from end to end (e.g. in a GSMIPSTN data call between the MS and the PSTN modem transparently through the IWF). This method has several drawbacks:                i) The error correction protocol has not necessarily been optimized for both legs. For example the error correction protocols V.42 and MNP4 supported by the modems are not optimal for a GSM traffic channel. The frame length is far longer than the frame length of the RLP. In deteriorating radio conditions, the probability of retransmission of such long frames is greater than that of the RLP frames, thus jamming the traffic channel effectively.        ii) If the end-to-end error correction and/or data compression negotiation fails, it is no longer possible to provide data compression for the legs. Assume for example that in a GSM/ISDN call, the transmission rate is 2*14.4 Kbps=28.8 Kbps on a GSM traffic channel (between the MS and the IWF) and 56 Kbps on an ISDN traffic channel (between the IWF and the ISDN terminal equipment). If end-to-end data compression can be negotiated, the uncompressed data rate might typically be 3*28.8 Kbps=about 90 Kbps. If the end-to-end compression negotiation fails and it is not possible to provide compression even on a leg, the data rate will be only 28.8 Kbps. If GSM compression could be negotiated between the IWF and the MS when the end-to-end compression negotiation fails, the end-to-end data rate would be the lower rate of the data rates of these two legs, e.g. 56 Kbps.        iii) The support of synchronous bearer services is required both in the mobile station and in the mobile network. For example in the GSM system, the availability of the synchronous bearer services is not as good as that of asynchronous bearer services. In practice, each GSM network and each mobile station that is capable of data transmission support the asynchronous bearer services.        
WO 94/05104 discloses a digital mobile system, wherein end-to-end compression is used between a mobile station MS and a PSTN modem, but a different error correction protocol is used on a traffic channel of the mobile network and on a modem connection. The digital data link between the MS and the IWF modem over the radio path is a non-transparent asynchronous data link where it is possible to use automatically the error protection protocol (e.g. the RLP) of the radio system that has been optimized to correct errors over the radio link. The error correction required on the modem connection is obtained by providing the IWF modem with an error correction protocol that is similar to the one in the PSTN modem situated at the other end of the modem connection. The data compression functions are thus provided in the mobile system in the MS and the error correction of the modem connection is provided in the IWF modem that does not participate in any way in the data compression. However, at the beginning of the modem connection the IWF modem negotiates, by means of a handshaking carried out with the PSTN modem, the compression parameters to be used in the data transmission and forwards them to the MS.
WO 94/05104 solves some of the aforementioned problems, but it has other drawbacks. Firstly, the method disclosed in WO 94/05104 requires non-standard operations. For example in the GSM system, the MS must interrupt the set-up of the RLP in order to wait for the compression parameters from the IWF. In addition, there must be a specific addressing mechanism that provides access to special functionality required by the method in the IWF. Secondly, the MS cannot participate in the negotiation for the compression parameters, and therefore optimum conditions (the best common group of parameter values) cannot always be reached. Thirdly, this known method has no fallback possibility, i.e. if end-to-end data compression cannot be set up, it is no longer possible to provide data compression on a leg (e.g. between the MS and the IWF).
Similar problems also occur in interfaces between other telecommunication networks.